外文翻译--关于自动化立体仓库使用双货叉问题的探讨.doc
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附录一外文翻译AnAnalysisOfDualShuttleAutomatedStorage/RetrievalSystemsBrettA.PetersAugust1,1994AbstractThispaperaddressesthethroughputimprovementpossiblewiththeuseofadualshuttleautomatedstorageandretrievalsystem.Withtheuseofsuchasystem,travelbetweentimeinadualcommandcycleisvirtuallyeliminatedresultinginalargethroughputimprovement.Thedualshuttlesystemisthenextendedtoperformanequivalentoftwodualcommandsinonecycleinaquadruplecommandmode(QC).AheuristicthatsequencesretrievalstominimizetraveltimeinQCmodeisdeveloped.MonteCarlosimulationresultsareprovidedforevaluatingtheheuristic'sperformanceandshowthatitperformswell,achievinglargethroughputimprovementscomparedwiththatofthedualcommandcycleoperatingunderthenearestneighborretrievalsequencingheuristic.Keywords:AutomatedStorage/RetrievalSystemsDesign;AutomatedStorage/RetrievalSystemsOperation;MaterialHandlingSystems;PerformanceModelingandAnalysisIntroductionAutomatedstorage/retrievalsystems(AS/RS)arewidelyusedinwarehousingandmanufacturingapplications.AtypicalunitloadAS/RSconsistsofstorageracks,S/Rmachines,linkconveyors,andinput/output(I/O)stations.Animportantsystemperformancemeasureisthethroughputcapacityofthesystem.Thethroughputcapacityforasingleaisleistheinverseofthemeantransactiontime,whichistheexpectedamountoftimerequiredfortheS/Rmachinetostoreand/orretrieveaunitload.TheservicetimeforatransactionincludesbothS/Rmachinetraveltimeandpickup/deposittime.ThistimetypicallydependsontheconfigurationofthestoragerackandtheS/Rmachinespecifications.Hanetal.[2]improvedthethroughputcapacityoftheAS/RSthroughsequencingretrievals.IntelligentlysequencingtheretrievalscanreduceunproductivetravelbetweentimewhentheS/Rmachineistravelingemptyandtherebyincreasethethroughput.TheydevelopanexpressionforthemaximumpossibleimprovementinthroughputiftravelbetweeniseliminatedforanAS/RSthatisthroughputboundandoperatesindualcommandmode.Inessence,thismeansthatiftheS/Rmachinetravelsinasinglecommandpathbutperformsbothastorageandaretrievaloperation,theabovethroughputimprovementcouldbeobtained.Inthispaper,weanalyzeanalternativedesignoftheS/RmachinethathastwoshuttlesinsteadofoneasinaregularAS/RS.Thenewdesigneliminatesthetravelbetweenthestorageandretrievalpointsandperformsbothastorageandaretrievalatthepointofretrieval,therebyachievingthemaximumthroughputincreasecalculatedbyHanetal.[3].ThedualshuttleAS/RSisanewdesignaimedatimprovingS/Rmachineperformance.moststudiesonAS/RSsystemshavebeenbasedonasingleshuttledesign.InouranalysisofthedualshuttleAS/RSperformance,webuilduponthesepreviousresearchresults.AlternativeS/RMachineDesignAtypicalunit-loadAS/RShasanS/Rmachineoperatingineachaisleofthesystem.TheS/Rmachinehasamastwhichissupportedatthefloorandtheceilingandtravelshorizontallywithintheaisle.Connectedtothismastisashuttlemechanismthatcarriestheunitloadandmovesverticallyupanddownthemast.Theshuttlemechanismalsotransfersloadsinandoutofstoragelocationsintherack.Figure1providesanillustrationofthesingleshuttleS/Rmachine.Figure1.SingleShuttleS/RMachineDesignAtypicalsingleshuttleAS/RScanperformasinglecommandcycleoradualcommandcycle.Asinglecommandcycleconsistsofeitherastorageoraretrieval.Forastorage,thetimeconsistsofthetimetopickuptheloadattheI/Opoint,traveltothestoragepoint,deposittheloadatthatpoint,andreturntotheI/Opoint.Thetimeforaretrievalisdevelopedsimilarly.Adualcommandcycleinvolvesbothastorageandaretrievalinthesamecycle.ThecycletimeinvolvesthetimetopickuptheloadattheI/Opoint,traveltothestoragelocation,placetheloadintherack,travelemptytotheretrievallocation,retrieveaload,returntotheI/Opoint,anddeposittheloadattheI/Opoint.IfwecriticallyanalyzethedualcommandcycleoftheS/Rmachine(shownbythesolidlineinFigure2),apotentialopenlocationforafuturestorageiscreatedwhenaretrievalisperformed.Furthermore,ifbotharetrievalandastorageareperformedatthesamepoint,thetravelbetweentime(TB)iseliminated,andthetraveltimewillbeequaltothesinglecommandtraveltime.WiththeexistingAS/RSdesign,thismodeofoperationisnotpossible;therefore,analternativetotheS/Rmachine,adualshuttleR/Smachine,isproposed.Figure2.DualCommandTravelPathsofS/RandR/SMachinesR/SMachineOperationConsideranS/Rmachinewithtwoshuttlemechanismsinsteadofone.ThisnewS/Rmachinecouldnowcarrytwoloadssimultaneously.Eachshuttlemechanismcouldoperateindependentlyoftheother,sothatindividualloadscanstillbestoredandretrieved.AnillustrationofthedualshuttleS/RmachineisshowninFigure3.ThisnewS/Rmachinewouldoperateasdescribedbelow.Figure3.DualShuttleS/RMachineDesignTheS/RmachinepicksuptheitemtobestoredfromtheI/Opoint,loadsitintothefirstshuttle,andmovestotheretrievallocation.Afterreachingtheretrievallocation,thesecondshuttleispositionedtopickuptheitemtoberetrieved.Afterretrieval,theS/Rmachinepositionsthefirstshuttleanddepositstheload.TheS/RmachinethenreturnstotheI/Opoint.TheoperationcaneasilybeseenasasinglecommandoperationplusasmalltraveltimeforrepositioningtheS/Rmachinebetweentheretrievalandstorage(aswellastheadditionalpickupanddeposittimeassociatedwiththesecondload).Therefore,theS/RmachinenowoperatesasanR/Smachineperformingaretrievalfirstthenastorageinadualcommandcycle.SincetheR/Smachinehastwoshuttles,thepositionoftheshuttleshasaroleintheoperationofthesystem.Withtwoshuttles,theR/Smachineisabletoperformadualcommandcycleatonelocationintherack.Thisoperationisaccomplishedbyfirstretrievingtheloadontotheemptyshuttle,transferringthesecondshuttleintoposition,andstoringtheloadintotheemptylocationintherack.However,thechoiceofshuttleconfigurationdoesnotimpacttheanalysisinthispaper.Toperformtheseoperations,theR/Smachinemustmovethesecondshuttleintopositionafterthefirstshuttlehascompletedtheretrieval.Duetothesmalldistanceinvolved,theR/Smachinewilluseaslowercreepspeedforpositioning,butthistraveltimeisgenerallysmall.Furthermore,anamountofcreeptimeisusuallyincludedinthepickupanddeposittimetoaccountforthisrequiredpositioning.AseconddesigncharacteristicisthatadditionalclearancebeyondthefirstandlastrowandcolumnoftherackmustbeprovidedforovertraveloftheR/Smachinetoaccommodatebothshuttlemechanisms.ThroughputImprovementToestimatethethroughputimprovementbythedualshuttlesystemoverexistingdesigns,weusetheexpressionsforsinglecommandanddualcommandcycletimesdevelopedbyBozerandWhite[1]andthetabulatedvaluesforthenearestneighborheuristicfromHanetal.[4].Indevelopingtheexpressions,theauthorsin[1]and[4]madeseveralassumptions.Thesameassumptionsholdforthenewdesignandincludethefollowing.1.TherackisconsideredtobeacontinuousrectangularpickfacewheretheI/Opointislocatedatthelowerleft-handcorneroftherack.2.Theracklengthandheight,aswellastheS/Rmachinevelocityinthehorizontalandverticaldirections,areknown.3.TheS/Rmachinetravelssimultaneouslyinthehorizontalandverticaldirections.Incalculatingthetraveltime,constantvelocitiesareusedforhorizontalandverticaltravel.Accelerationanddecelerationeffectsare
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